Grain Growth in Si3 N4-Based Materials

Abstract

The growth of Si3N4 grains in the usual sintering process occurs in an oxynitride liquid formed by reactions between sintering additives, impurity SiO2, and Si3N4. The shape of an Si3N4 grain in the liquid matrix is a hexagonal rod, although the aspect ratio (c/a) varies considerably, depending on the processing conditions and final crystalline forms of α or β. Figure 1 shows two types of microstructures observed in sintered Si3N4-based materials. The microstructure shown in Figure la is normal with unimodal grain-size distribution and that of Figure 1b is abnormal with a microstructure of exceptionally large elongated grains within fine matrix grains. When normal grain growth occurs, the microstructure varies little with sintering time, and the development may be described by a simple law. But, when abnormal grain growth occurs, a duplex grain structure with a bimodal grainsize distribution results; then no simple kinetic law can describe the microstructure development.The grain growth in the microstructures shown in Figure 1 exemplifies the growth of faceted grains in a liquid matrix. The grain growth in a matrix occurs via growth of larger grains and dissolution of smaller ones by material transport through the liquid phase. The driving force of the material transport for an individual grain is determined by the difference between its size and the critical grain size, which is invariant at the moment of observation. Since the driving force is usually low, the volume change of each grain is relatively slow and analogous to the crystal growth or dissolution in a liquid matrix under low super- or undersaturation. Therefore, knowledge of the growth behavior of faceted crystals under low supersaturation in a liquid may provide fundamental understanding of the grain growth in Si3N4-based materials.